MacArthur & Wilson’s (1967) classic theory of island biogeography predicts that island species richness arises from an equilibrium between immigration and extinction, with islands further from a mainland receiving fewer immigrants and thus having lower species richness. A twist on the basic paradigm occurs for land-bridge islands, i.e., islands that were previously connected to a mainland. Such islands may exhibit more species than expected if insufficient time has passed for the immigration–extinction equilibrium to be reached. For example, in Baja California more lizard species are found on islands that have become isolated more recently in geological time (Wilcox 1978).
In a paper just published in the Journal of Biogeography, we explored whether this effect exists for birds on the islands of Sundaland. The project was led by Keita Sin, a former Honours student in Frank Rheindt’s lab, who created a database of bird diversity on 94 of these islands, from both published inventories and his own field work. We tested whether shelf islands, which have been connected to major landmasses at various points over the last 20,000 years, have higher bird diversity than do oceanic islands. Surprisingly, we found that they do not. Our explanation is that both immigration and extinction rates for birds on islands are higher than for most other taxa, such as lizards and plants, and thus the immigration–extinction equilibrium is reached faster. Immigration rates are higher because birds are effective dispersers; extinction rates may be higher because bird species’ population sizes on islands are typically small. Our results help shed light on the processes that structure species diversity on islands.
Chisholm Lab 